hi kitsunechanw, the error message is unfortunately misleading in this case - what it's really meaning is that the addon's signature doesn't check out (probably mistakenly)...

i assume you've already updated to firefox 66.0.4 - if so , the most common case of users still having troubles there that i've seen is when a master password was enabled long ago. please refer to https://support.mozilla.org/en-US/kb/add-ons-disabled-or-fail-to-install-firefox#w_master-password and see if the workaround is applicable for you.

thanks i removed my master password and it worked

great, thanks for following up. this fix just needed to run once, so please feel free to reset a master password again afterwards if you wish to do so.